Incremental transfer of database segments

A non-transitory machine-readable medium storing a program which when executed by at least one processing unit migrates data from a first database to a second database, the data susceptible to change at the first database before all of the data is migrated to the second database and is available for access through the second database, the program comprising sets of instructions for:

• transferring a plurality of segments of data from the first database to the second database and generating a first digital signature for each transferred segment;
• iteratively:
• identifying a set of segments that have been modified in the first database since the last transfer of one or more segments to the second database, before the data is available for access through the second database, said identifying comprising generating a second digital signature for each transferred segment in the set and comparing the second digital signature with a prior digital signature that was last generated for the transferred segment;
• and
• transferring the set of identified segments from the first database to the second database, each transferred, identified segment to replace a previously transferred segment,
• said iterations terminating after a determination that no segment in the first database has been modified since the last transfer of one or more segments to the second database, such that all the segments stored in the second database now match all of the transferred segments in the first database.